1. Technical Field
The present invention pertains to an antenna; more particularly the present invention pertains to a left-hand circular polarized GPS antenna used to receive space-based satellite GPS signals after reflecting off of a surface an odd number of times.
2. History of Related Art
Polarization is a description of how the direction of the electric field vector changes within an electromagnetic wave at a fixed point in space over time. If the wave is propagating in the positive z-direction, the electric field vector at a fixed point, for example at z=0.0, can be expressed in the following general form: EQU E.sub.z=0,t =.delta..sub.x E.sub.o cos(.omega.t)+.delta..sub.y AE.sub.o cos(.omega.t+.phi.)
Mathematically, linear and circular polarization are special cases of elliptical polarization. Consider two electric-field vectors at right angles to each other propogating in the same direction. The frequencies are the same, but the magnitudes and face angles vary. If either one or the other of the magnitudes is zero, linear polarization results. If the magnitudes are the same and the phase angle between the two vectors (in time) is 90 degrees, circular polarization results. Of course, any combination between these two limits gives elliptical polarization.
The ideal antenna for use with random polarization is one with a circularly polarized radiation pattern. Polarization sense is a critical factor, especially when satellites are used to propagate signals, since the receiving antenna must be of the same polarity as the transmitting antenna for proper reception. In the case of GPS satellites, the most common transmitted signal is the right hand circular polarized signal. This occurs when the values for the general equation above include A=1 and .phi.=-.pi./2, thus: EQU E.sub.z=0,t =.delta..sub.x E.sub.o cos(.omega.t)+.delta..sub.y E.sub.o cos(.omega.t-.pi./2)
The x and y components of the electrical field in this case have the same magnitude, and oscillate 90 degrees out of phase.
The signal emanating from the space-based satellite GPS system is right-hand circular polarized, and is intended to be received by a Right-Hand Circular Polarized (RHCP) antenna. However, optimal reception of a RHCP signal by a RHCP antenna requires that the antenna be in direct line-of-sight with the satellite. If the RHCP signal reflects off of a surface before striking the antenna, the polarity will be reversed (to Left-Hand Circular Polarized (LHCP)) with an attendant loss of signal strength.
The characteristic equation for a Left-Hand Circularly Polarized signal results when A=1 and .phi.=.pi./2, thus: EQU E.sub.z=0,t =.delta..sub.x E.sub.o cos(wt)+.delta..sub.y E.sub.o cos(.omega.t+.pi./2).
Thus, the LHCP signal is 180 degrees out of phase with the RHCP signal, which gives at least a 3.0 dB signal loss in practice. If the receiver is sensitive, this may not be a problem. However, for many applications, it is desirable to reduce the amount of receiver sensitivity needed so as to enhance the signal-to-noise ratio. Further, a less sensitive receiver is less expensive to manufacture. Also, many applications utilizing GPS technology simply cannot physically locate the receiving antenna such that a direct line-of-sight with the satellite transmitting the RHCP signal is possible.
Since some applications utilizing GPS technology must position the receiving antenna such that signal reflection is necessary, an antenna is needed which can make the best use of a reflected signal. In addition, a method of using the antenna to best make use of such a reflected RHCP signal is needed.